As is well known in the gas turbine engine art, it is abundantly important to utilize engine cooling air in the most expeditious manner inasmuch as its use results in a penalty in engine performance. Hence, to minimize its use and maximize engine performance, it becomes paramount that designers of gas turbine engines obtain the maximum cooling effectiveness with minimum pressure drop requirements and cooling air flow rates.
Cooled turbine blades of the type that flows air internally typically bleed air from the engine's compressor section into three major portions: the trailing edge, the leading edge, and the middle section therebetween. Inasmuch as this invention deals solely with the trailing edge, for the sake of simplicity and convenience, only the trailing edge will be considered and described herein. Specifically, the trailing edge as considered herein is that portion of the airfoil that is aft of the passage channeling the cooling air up from the root of the blade.
Historically, trailing edges of airfoils have heretofore been cooled using combinations of features such as pedestals, impingement rows, slots, trip strips and dimples. An understanding of the prior art can be had by referring to the turbine blade depicted in FIGS. 1 and 2.
The cooling air flowing up the supply passage 10 is bled through a row of impingement holes 12. Then the air, now flowing in a primarily axial direction with respect to the engine centerline, is bled through a second row of impingement holes 13. Obviously, total pressure of the cooling air is reduced across each row of impingement holes. At the same time that coolant pressure is being reduced, external gaspath pressure in which the turbine is operating is also declining as the gas accelerates in the converging airfoil passages 14. Coolant pressure in the internal passages is always maintained at a higher level of pressure than external gaspath pressure to ensure the ability to insert film cooling holes into the passages, or to ensure outflow of coolant in the event a crack is created through the wall. The chambers directly behind the impingement rows allow radial flow, preventing local blockages due to imperfect castings or foreign material from causing an extended hot streak all the way to the trailing edge. After passing through the second row of impingement holes and collecting in the second chamber 15, the cooling air enters slots 16 which conduct the air to discharge ports 17 on the concave side of the airfoil just forward of the extreme trailing edge 18. As the air passes through these impingement rows and slots, high levels of heat transfer are generated on the internal walls due to boundary layer disturbances created by impingement and entrances.
Alternative geometries to the one described above are commonly in use Specific applications dictate in many instances the types of features which provide the most advantage. Certain applications call for multiple rows of pedestals which provide good heat transfer with lower pressure drop than impingement rows. Trip strips of various shapes and sizes are commonly used in conjunction with impingement rows and pedestals, with and without slots. All these approaches are similar in that they augment heat transfer coefficients and surface area through a series of contractions, moving the flow in the axial direction, while allowing radial communication.
Typically, flow through the trailing edge is restricted as much as possible while still providing uniform cooling in the radial direction. Restriction is limited by the minimum allowable passage size, which is determined by producibility considerations. Small passages are created by small, fragile core features in the investment casting method now used almost exclusively in the manufacture of cooled turbine airfoils. When passages are driven to too small a size to restrict flow, they are prone to breakage due to handling and stresses induced during the manufacturing process.
We have found that we can enhance cooling effectiveness without increasing flow levels and without the utilization of the heat transfer enhancement techniques described immediately above. This invention contemplates utilizing a cascade formed of rows of staggered turning vanes or ribs. Not only does this inventive concept afford a high cooling effectiveness at a given cooling flow level, it also provides improved producibility in the manufacturing of turbine blades made in production.